pragma solidity ^0.4.12;

/**
 * @title SafeMath
 * @dev Math operations with safety checks that throw on error
 */

library SafeMath {

  function mul(uint256 a, uint256 b) internal constant returns (uint256) {
    uint256 c = a * b;
    assert(a == 0 || c / a == b);
    return c;
  }

  function div(uint256 a, uint256 b) internal constant returns (uint256) {
    // assert(b > 0); // Solidity automatically throws when dividing by 0
    uint256 c = a / b;
    // assert(a == b * c + a % b); // There is no case in which this doesn't hold
    return c;
  }

  function sub(uint256 a, uint256 b) internal constant returns (uint256) {
    assert(b <= a);
    return a - b;
  }

  function add(uint256 a, uint256 b) internal constant returns (uint256) {
    uint256 c = a + b;
    assert(c >= a);
    return c;
  }

}

/**
 * @title Ownable
 * @dev The Ownable contract has an owner address, and provides basic authorization control
 * functions, this simplifies the implementation of "user permissions".
 */
contract Ownable {

  address public owner;

  event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);

  /**
   * @dev The Ownable constructor sets the original `owner` of the contract to the sender
   * account.
   */

  function Ownable() {
    owner = msg.sender;
  }

  /**
   * @dev Throws if called by any account other than the owner.
   */

  modifier onlyOwner() {
    require(msg.sender == owner);
    _;
  }

  /**
   * @dev Allows the current owner to transfer control of the contract to a newOwner.
   * @param newOwner The address to transfer ownership to.
   */

  function transferOwnership(address newOwner) onlyOwner public {
    require(newOwner != address(0));
    OwnershipTransferred(owner, newOwner);
    owner = newOwner;
  }

}

/**
 * @title ERC20Basic
 * @dev Simpler version of ERC20 interface
 * @dev see https://github.com/ethereum/EIPs/issues/179
 */

contract ERC20Basic {

  uint256 public totalSupply;

  function balanceOf(address who) public constant returns (uint256);
  function transfer(address to, uint256 value) public returns (bool);
  event Transfer(address indexed from, address indexed to, uint256 value);

}

/**
 * @title Basic token
 * @dev Basic version of StandardToken, with no allowances.
 */

contract BasicToken is ERC20Basic, Ownable {

  using SafeMath for uint256;

  mapping(address => uint256) balances;

  // allowedAddresses will be able to transfer even when locked
  // lockedAddresses will *not* be able to transfer even when *not locked*

  mapping(address => bool) public allowedAddresses;
  mapping(address => bool) public lockedAddresses;
  bool public locked = true;

  function allowAddress(address _addr, bool _allowed) public onlyOwner {
    require(_addr != owner);
    allowedAddresses[_addr] = _allowed;
  }

  function lockAddress(address _addr, bool _locked) public onlyOwner {
    require(_addr != owner);
    lockedAddresses[_addr] = _locked;
  }

  function setLocked(bool _locked) public onlyOwner {
    locked = _locked;
  }

  function canTransfer(address _addr) public constant returns (bool) {
    if(locked){
      if(!allowedAddresses[_addr]&&_addr!=owner) return false;
    }else if(lockedAddresses[_addr]) return false;

    return true;
  }

  /**
  * @dev transfer token for a specified address
  * @param _to The address to transfer to.
  * @param _value The amount to be transferred.
  */

  function transfer(address _to, uint256 _value) public returns (bool) {
    require(_to != address(0));
    require(canTransfer(msg.sender));

    // SafeMath.sub will throw if there is not enough balance.

    balances[msg.sender] = balances[msg.sender].sub(_value);
    balances[_to] = balances[_to].add(_value);
    Transfer(msg.sender, _to, _value);

    return true;
  }

  /**
  * @dev Gets the balance of the specified address.
  * @param _owner The address to query the the balance of.
  * @return An uint256 representing the amount owned by the passed address.
  */

  function balanceOf(address _owner) public constant returns (uint256 balance) {
    return balances[_owner];
  }
  
}

/**
 * @title ERC20 interface
 * @dev see https://github.com/ethereum/EIPs/issues/20
 */

contract ERC20 is ERC20Basic {
  function allowance(address owner, address spender) public constant returns (uint256);
  function transferFrom(address from, address to, uint256 value) public returns (bool);
  function approve(address spender, uint256 value) public returns (bool);
  event Approval(address indexed owner, address indexed spender, uint256 value);
}

/**
 * @title Standard ERC20 token
 *
 * @dev Implementation of the basic standard token.
 * @dev https://github.com/ethereum/EIPs/issues/20
 * @dev Based on code by FirstBlood: https://github.com/Firstbloodio/token/blob/master/smart_contract/FirstBloodToken.sol
 */

contract StandardToken is ERC20, BasicToken {

  mapping (address => mapping (address => uint256)) allowed;

  /**
   * @dev Transfer tokens from one address to another
   * @param _from address The address which you want to send tokens from
   * @param _to address The address which you want to transfer to
   * @param _value uint256 the amount of tokens to be transferred
   */

  function transferFrom(address _from, address _to, uint256 _value) public returns (bool) {
    require(_to != address(0));
    require(canTransfer(_from));

    uint256 _allowance = allowed[_from][msg.sender];

    // Check is not needed because sub(_allowance, _value) will already throw if this condition is not met
    // require (_value <= _allowance);

    balances[_from] = balances[_from].sub(_value);
    balances[_to] = balances[_to].add(_value);
    allowed[_from][msg.sender] = _allowance.sub(_value);
    Transfer(_from, _to, _value);

    return true;

  }

  /**
   * @dev Approve the passed address to spend the specified amount of tokens on behalf of msg.sender.
   *
   * Beware that changing an allowance with this method brings the risk that someone may use both the old
   * and the new allowance by unfortunate transaction ordering. One possible solution to mitigate this
   * race condition is to first reduce the spender's allowance to 0 and set the desired value afterwards:
   * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
   * @param _spender The address which will spend the funds.
   * @param _value The amount of tokens to be spent.
   */

  function approve(address _spender, uint256 _value) public returns (bool) {
      
    allowed[msg.sender][_spender] = _value;
    Approval(msg.sender, _spender, _value);
    return true;
    
  }

  /**
   * @dev Function to check the amount of tokens that an owner allowed to a spender.
   * @param _owner address The address which owns the funds.
   * @param _spender address The address which will spend the funds.
   * @return A uint256 specifying the amount of tokens still available for the spender.
   */

  function allowance(address _owner, address _spender) public constant returns (uint256 remaining) {
    return allowed[_owner][_spender];
  }

  /**
   * approve should be called when allowed[_spender] == 0. To increment
   * allowed value is better to use this function to avoid 2 calls (and wait until
   * the first transaction is mined)
   * From MonolithDAO Token.sol
   */

  function increaseApproval (address _spender, uint _addedValue)
    returns (bool success) {
    allowed[msg.sender][_spender] = allowed[msg.sender][_spender].add(_addedValue);
    Approval(msg.sender, _spender, allowed[msg.sender][_spender]);
    return true;
  }

  function decreaseApproval (address _spender, uint _subtractedValue)
    returns (bool success) {
    uint oldValue = allowed[msg.sender][_spender];
    if (_subtractedValue > oldValue) {
      allowed[msg.sender][_spender] = 0;
    } else {
      allowed[msg.sender][_spender] = oldValue.sub(_subtractedValue);
    }
    Approval(msg.sender, _spender, allowed[msg.sender][_spender]);
    return true;
  }

}


/**
 * @title Burnable Token
 * @dev Token that can be irreversibly burned (destroyed).
 */

contract BurnableToken is StandardToken {

    event Burn(address indexed burner, uint256 value);
    
    /**
     * @dev Burns a specific amount of tokens.
     * @param _value The amount of token to be burned.
     */

    function burn(uint256 _value) onlyOwner public {
        
        require(_value > 0);
        require(_value <= balances[msg.sender]);

        // no need to require value <= totalSupply, since that would imply the
        // sender's balance is greater than the totalSupply, which *should* be an assertion failure

        address burner = msg.sender;
        balances[burner] = balances[burner].sub(_value);
        totalSupply = totalSupply.sub(_value);
        Burn(burner, _value);
        Transfer(burner, address(0), _value);
        
    }


}

 

contract Token is BurnableToken {

    string public constant name = "Gem Exchange and Trading";
    string public constant symbol = "GXT";
    uint public constant decimals = 18;

    // there is no problem in using * here instead of .mul()

    uint256 public constant initialSupply = 500000000 * (10 ** uint256(decimals));

    // Constructors

    function Token () {
        totalSupply = initialSupply;
        balances[msg.sender] = initialSupply; // Send all tokens to owner
        allowedAddresses[owner] = true;
    }

}